In the printing industry, it is common practice for a customer to provide a digital image to a printer, who then provides a sample printed version of that image to the customer for approval prior to printing a large number of copies of the image using a high volume output device such as a printing press. The sample printed version is known as a “proof” and it is used to ensure that the customer is satisfied with the contents, composition and color gamut and tone characteristics of the image.
It is not cost effective to print the proof using a high volume output device since it is expensive to set up. Accordingly, it has become the practice in the printing industry to use digital color printers to print proofs. The digital color printer is configured so that the printed version of an image is close in color to what one would obtain on the high volume output device, and this printed image is typically presented to the customer for approval before the digital image is printed on the high volume output device.
In some situations, the customer may provide to the printer not only the digital image of an image to be printed in volume, but also a reference image, with a demand that the printer should match their proof and printed copies of the image to the reference image. This creates a considerable problem, since the printer has to now render printed images from the digital image in such a way that the printed images match the customer-provided reference image. Printers typically do not have transformation information that would allow printing equipment to match the customer reference image. In some cases, the customer-provided reference image may be similar to the proof that the printer makes. Consequently, there may be enough latitude in the configuration of the high volume output device to allow the device to be adjusted to match the customer-provided reference image. In other cases, the customer-provided reference image may not be within the range of adjustment of the high volume output device. In yet other cases, it may be that multiple images need to be printed simultaneously on a single “flat” on the high volume output device, and the adjustments necessary to make the output from the high volume output device match the customer-provided reference image would cause other images on the flat to not match their corresponding printer-generated proof or customer-provided reference image.
One approach to this problem has been to use tonal adjustments on each ink separately, as is typically done to address the problem of dot gain. In general, however, this requires that such gain be applied to individual images rather than an entire flat of images that will be printed simultaneously, and not all digital printing workflow software allows such image-specific adjustments. In practice, the necessary tonal adjustments are usually estimated subjectively rather than from measurements, with the consequence that if the adjustment does not provide an adequate color match then the adjustments must themselves be adjusted. The resulting iterative process is therefore costly and time-consuming. Furthermore, it is often the case that a good color match cannot be achieved by adjusting individual inks separately.
A more typical approach to addressing this problem is for the printer first to generate his own proof on his own proof printer, compare to the customer-provided reference image, and adjust the digital image manually, using image manipulation software such as Adobe Photoshop, and repeat this process until the proof matches the customer-provided reference image. Since his volume printers will color-match his proof-printer, he knows that if his proof color-matches the reference image, his volume printers will print images that are color-matched to the reference image. Since the reference image may in fact have colors that differ significantly from the intended standard, such as SWOP, or from the colors that the printer's proof printer produces for the image, this iterative process also often turns out to be costly and time-consuming.
There is therefore a need for an automated method by which a digital image provided by a customer to a printer may be modified by the printer, based on a supplied reference image produced from the same digital image, so that the modified digital image, when printed by the printer's high volume output device, will color-match the customer-provided reference image.